Conventionally, as a system where a plurality of slave station apparatuses commonly use a transmission medium and a transmission band and the respective slave station apparatuses transmit data to a host station apparatus according to band control by the host station apparatus, for example, an optical network having n-numbered ONUs (Optical Network Units) and one OLT (Optical Distribution Termination) shown in FIG. 5/G.983.1-Generic physical configuration of the Optical Distribution Network of Recommendation G. 983. 1 (Broadband optical access systems based on Passive Optical Networks (PON) 1998/10) is known.
FIG. 7 is a block diagram showing a schematic structure of the above-mentioned optical network. In FIG. 7, OLT is a host station apparatus, and a plurality of ONUs are slave station apparatuses. Moreover, ODN (Optical Distribution Network) is a transmission medium composed of an optical fiber, an optical multiplexer/branching filter and the like. FIG. 8 is a diagram showing formats of data in a direction from OLT side to ONU side (downstream) and data in a direction from ONU side to OLT side (upstream) shown in ITU-T Recommendation FIG. 11/G. 983. 1-Frame format for 155.52/155.52 Mbit/s PON. As shown in FIG. 8, the downstream data are composed of a fixed length cell of 53 bytes, and the upstream data are composed of a fixed length cell of 56 bytes. The downstream data are multiple-addressed from one OLT to all ONUs, and as for the upstream data, data transmission to one ONU is allowed to be transmitted by OLT for each time slot of each cell.
The permission of the data transmission for each upstream time slot, namely, the post of the band allocation information is defined by the format of the downstream data, and is inserted into a monitor and control (PLOAM: Physical Layer Operations Administration and Maintenance) cell which is transmitted in a cycle of 28 cells. This band allocation information is inserted as each byte of “GRANT 1” through “GRANT 27” of PLOAM cell as described in Table 8/G. 983. 1-Payload content of downstream PLOAM cell of ITU-T Recommendation. As shown in FIG. 8, since the upstream data exist in 53 slots for each frame, “GRANT 1” through “GRANT 27” which are inserted in the first PLOAM cell in the downstream frame show band allocation information for the first through 27th time slots in the upstream frame, and “GRANT 1” through “GRANT 27” which are inserted in the second PLOAM cell in the downstream frame show band allocation information for the 28th through 53rd time slots in the upstream frame. Namely, the allocation to the 53 time slots in the upstream frame is instructed by “GRANT 1” through “GRANT 27” which are inserted in the two PLOAM cells in the downstream frame.
The format of “GRANT” is shown in Table 10/G. 983. 1-Specification of the grants in ITU-T Recommendation. In “Table”, a type of “GRANT” allocated to use of the upstream slots of ONUs is “DataGRANT” or “PLOAMGRANT”. “PLOAMGRANT” is allocated to transmission of PLOAM cell in the upstream direction, and for allocation for transmission of normal data, “Data GRANT” is used. A value to be used in “Data GRANT” is arbitrary except for partial reserved values. Values of “Data GRANT” to be previously used by respective ONUs as well as values of “PLOAM GRANT” are posted to the respective ONUs from OLT by messages included in the downstream PLOAM cells in order that the respective ONUs themselves recognize the band allocation. The formats of the messages are shown in Grant allocation message in Downstream message formats ITU-T Recommendation, 8.3.8.2.1. One of these messages is transmitted to ONU, and a value of “Data GRANT” and a value of “PLOAM GRANT” to be used by this ONU are shown. The ONU receives this message and stores the value as initial setting, and as a result, in the case where stored “GRANT” exists in “GRANT 1” through “GRANT 27” in the PLOAM cells transmitted from OLT, the ONU recognizes that band allocation of the upstream time slot exists for the ONU itself.
In such a manner, OLT sets “Data GRANT” and “PLOAM” which are individual values for the respective ONUs, and before the respective ONUs transmit upstream data, they transmit messages so as to manage the band allocation for the respective ONU in the upstream slots. ONU does not transmit data in a slot where band allocation to this ONU does not exist so as to prevent conflict of data in an upstream transmission line.
Incidentally, in the optical network, since a type of data transmitted by ONU cannot be discriminated, services stored in ONU cannot be occasionally satisfied sufficiently. For example, in the case where periodic data such as sound data which require periodic transmission upon real-time request and burst data which are transmitted in a burst manner such as file transmission between computers coexist, it is occasionally difficult to hold the periodic transmission of the periodic data such as sound data securely.
With reference to FIGS. 9 and 10, there will be explained below the case where the periodic transmission of the periodic data becomes difficult. FIG. 9 is a block diagram showing a structure of a conventional optical burst transmission/reception system, and FIG. 10 is a time chart showing a data transmission state in the case where ONU (slave station apparatus) stores a service which requires data transmission of the periodic data and a service which requires data transmission of the burst data.
In FIG. 9, a host station apparatus 10 is connected to a plurality of slave station apparatuses 20-1 through 20-n via a main fiber 31, an optical splitter 30 and branch fibers 32-1 through 32-n. The optical splitter 30 branches an optical signal from the host station apparatus 10 and transmits the branched signals to the slave station apparatuses 20-1 through 20-n, and multiplexes the optical signals from the slave station apparatuses 20-1 through 20-n and transmits the multiplexed signal to the host station apparatus 10.
Firstly the host station apparatus 10 has a code allocation section 14, and the code allocation section 14 previously sets codes as values of “GRANT” for the respective slave station apparatuses 20-1 through 20-n, and transmits the values of “GRANT” to a management signal transmission section 12. The management signal transmission section 12 allows the input values of “GRANT” to be contained in a PLOAM cell (management signal) in a format that the values can be identified by the slave station apparatuses 20-1 through 20-n, and transmits the management signal to an optical transmitter-receiver 11. Here, as for a corresponding relationship between the slave station apparatuses 20-1 through 20-n and the values of “GRANT”, sets of preset identification numbers of the slave station apparatuses 20-1 through 20-n and the values of “GRANT” are posted from the host station apparatus 10 to the slave station apparatuses 20-1 through 20-n respectively.
The optical transmitter-receiver 11 converts the management signal into an optical signal, and transmit the optical signal to the optical splitter 30 via main fiber 31. The optical splitter 30 distributes the optical signal via the branch fibers 32-1 through 32-n so as to transmit it to the slave station apparatuses 20-1 through 20-n. Respective optical transmitter-receivers 21 in the slave station apparatuses 20-1 through 20-n convert input optical signals into electrical signals, and transmit them to at least code identification sections 22 and allocation identification sections 23. The code identification section 22 fetches a management signal from the input electrical signal, and fetches the identification number preset in its slave station apparatus and a value of “GRANT” relating to this identification number and stores them.
Thereafter, the band allocation section 13 of the host station apparatus 10 transmits the values of “GRANT” presets for the slave station apparatuses 20-1 through 20-n to the management signal transmission section 12 with frequency according to bands required by the slave station apparatuses 20-1 through 20-n, and the management signal transmission section 12 inserts the values of “GRANT” into the slot allocation areas in the upstream direction in the management signal, and multiple-addresses the management signal to the slave station apparatuses 20-1 through 20-n via the optical transmitter-receiver 11. In the case where the band allocation is large, a lot of areas containing codes (values of GRANT) of the slave station apparatuses with large band allocation appear in the slot allocation areas in the upstream direction, and in the case where the band allocation is small, areas containing codes of the slave station apparatuses with small band allocation are less in the slot allocation areas in the upstream direction. Namely, an interval of appearance of the codes allocated to the slave station apparatuses change due to the band allocation. The band allocation is posted from the host station apparatus 10 to the slave station apparatuses 20-1 through 20-n repeatedly.
The code identification sections 22 of the slave station apparatuses 20-1 through 20-n detect the values of “GRANT” in the management signal and posts them to data reading sections 24. The data reading sections 24 check the values in the allocation areas in the respective slot input from the allocation identification sections 23, and when the values match with the codes posted from the code identification sections 22, the data reading sections 24 execute data reading process on the upstream time slots corresponding to the matched slots. This data reading process attempts to read the data from a buffer memory 25a, and when there exist no data to be read, the process reads data from a buffer memory 25b. The read data are multiplexed by a multiplexing section 27, and are transmitted to the data reading section 24. The data reading section 24 transmits the multiplexed data to the optical transmitter-receiver 21, and transmits them to the host station apparatus 10 via the branch fibers 32-1 through 32-n, the optical splitter 30 and the main fiber 31. When there exist no data to be read in the buffer memories 25a nor 25b, the data reading section 24 generates empty data so as to transmit them to the optical transmitter-receiver 21.
The data to be input into the buffer memory 25a of the slave station apparatus 20-1 are periodic data 26a, and the data to be input into the buffer memory 25b are burst data 26b.The data reading section 24 makes control so as to read the periodic data 26a in the buffer memory 25a in preference to the burst data 26b in the buffer memory 25b. This is because the burst data to be input in the burst manner do not normally require real-time property unlike sound data, and even if transmission is delayed to a certain extent, all of the burst data may be transmitted, but as the periodic data require real-time property, it is necessary that the periodic data have periodicity and are transmitted in a state that a delay is reduced as much as possible.
There will be explained below the data reading process in the case where the periodic data 26a (“a1” through “a4”) shown in FIG. 10(a) and the burst data 26b (“b1” through “b4”) shown in FIG. 10(b) are input respectively into the buffer memories 25a and 25b. The data reading section 24 reads the data from the buffer memories 25a and 25b with an interval equivalent to a sum of the bands required for transmission of the periodic data and the burst data (interval of data reading signals shown in FIG. 10(c)) correspondingly to the codes identified by the allocation identifying section 23 (band allocation information) . In this case, since the periodic data 26a are read in preference to the burst data 26b, as shown in FIG. 10(d), the read data, which are read by the data reading section 24 and are transmitted to the optical transmitter-receiver 21, are read with an interval equal with or double interval of a data reading signal shown in FIG. 10(c). As a result, the transmission interval of the periodic data 26a is not periodic. For example, the periodic data “a4” deviate from the periodicity.
In order to solve this problem, the band allocation section 13 of the host station apparatus 10 generates values of “GRANT” with the same period as the periodic data and additionally generates values of “GRANT” for the burst data in the burst manner, and the slave station apparatus 20-1 transmits their multiplexed result to the host station apparatus (see FIG. 11). In this case, the periodicity of the periodic data “a1” through “a4” is maintained. Particularly in the case where time difference between the periodic data to be input into the buffer memory 25a and “GRANT” generated for the periodic data, namely time difference between the periodic data and the data reading signal is small, the periodic data “a1” through “a4” transmitted from the slave station apparatus 20-1 keep the periodicity.
However, as shown in FIG. 12, the in the case where time difference between the periodic data input into the buffer memory 25a and the data reading signal is large, at the time of the reading by means of the data reading signal corresponding to “GRANT” allocated for the burst data, the input periodic data exist and they are read preferentially. For this reason, there arises a problem that the periodicity of the periodic data is not ipso facto maintained unlike the periodic data “a2” (see period Tv1 and period Tv2 in FIG. 12).
The difference between the system shown in FIG. 11 and the system shown in FIG. 12 is only a time relationship between the periodic data input into the slave station apparatus 20-1 and the value of “GRANT” generated from the host station apparatus 10 (data reading signal) . In this case, the host station apparatus 10 can recognize a slot interval Tc required for electric transfer of the periodic data, but difficultly recognizes timing at which the periodic data are input into the slave station apparatus 20-1. This is because the transmission time of the periodic data capable of being recognized by the host station apparatus 10 is only a slot position allocated by the host station apparatus 10 originally. For this reason, it is difficult to securely avoid collapse of the periodicity of the periodic data shown in FIG. 11.
Here, there considers a system which once stores data whose periodicity is collapsed into a buffer memory and reads periodic data from this buffer memory periodically so as to compensate the periodicity. However, in this case, there arises a problem that transmission delay due to the storage of the periodic data occurs and the original characteristic of the periodic data which requires real-time property cannot be kept securely.
Therefore, it is an object of the present invention to provide an optical burst transmission/reception control system in which in the case where periodic data and burst data are multiplexed so as to be transmitted from one slave station apparatus to a host station apparatus, periodicity of the periodic data is maintained and delay of the periodic data can be suppressed minimally, a host station apparatus and a slave station apparatus to be used in the system, and an optical burst transmission/reception control method.